The present invention relates to alarm systems and specifically to line security apparatus for alarm systems, particularly alarm systems of the direct current, direct wire type.
In a number of prior alarm systems, line security apparatus applies a square wave or sinusoidal security signal to one end of the communications line and detects whether the security signal is received at the opposite end. U.S. Pat. No. 3,786,501 to Marnerakis discloses a direct current alarm system having a sinusoidal security signal imposed on the direct current at the remote premises and apparatus at the monitoring station to detect the signal. In U.S. Pat. No. 3,641,547 to Reiss, et al., a detecting loop-type alarm system has line security apparatus which imposes at one end of the loop at the monitoring station a square wave which is periodically reduced in its positive amplitude. A detector at the monitoring station at the opposite end of the loop determines, during these periodic reductions, whether the security of the detecting loop has been breached by insertion of excessive resistance.
Circuitry for producing random patterns of digital bits is known to persons skilled in digital electronics.